Update widget catalog to show Material 3 widgets (#8574)

Fixes #8432. 

Site changes are viewable at the staging site:
https://flutter-site-73ed1.web.app/development/ui/widgets/.

Primary changes: 
- Addition of Material 3 Components card
[(view)](https://flutter-site-73ed1.web.app/development/ui/widgets/).
- New Material 3 page showing M3 widgets as displayed in matching
categories to material.io/components. This also includes a note about
Material 3 becoming the default - this text is not final and can be
iterated on in review.
- Widget cards in the M3 page have a hover effect applied. 
- In the widgets overview page, Material now links to M3, and contains a
link to the previous M2 widgets page.

General notes:
- Material 2 page ~~remains unchanged~~ has a notice about Material 3.
- No light/dark modes - this was explored but decided against, with the
possibility of returning to it if the site undergoes a site-wide dark
mode addition.

## Presubmit checklist
- [x] This PR doesn’t contain automatically generated corrections
(Grammarly or similar).
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style) — for example, it
doesn’t use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first person).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/master/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
of 80 characters or fewer.

Author: Eilidh Southren

src/_data/catalog/index.json

@@ -6,8 +6,9 @@
     "id": "basics"
   },
   {
-    "name": "Material Components",
-    "description": "Visual, behavioral, and motion-rich widgets implementing the <a href=\"https://material.io/design/guidelines-overview\">Material Design guidelines</a>.",
+    "name": "Material 2 Components",
+    "description": "Widgets implementing the <a href=\"https://m2.material.io/design\">Material 2 Design guidelines</a>.",
+    "pagecontent": "Flutter will make Material 3 (M3), the latest version of Material Design, the default in late 2023. Check out our <a href=\"/development/ui/widgets/material/\">M3 widget catalog</a> now.",
     "subcategories": [
       {
         "name": "App structure and navigation"
@@ -28,6 +29,40 @@
         "name": "Layout"
       }
     ],
+    "id": "material2"
+  },  
+  {
+    "name": "Material Components",
+    "description": "Visual, behavioral, and motion-rich widgets implementing the <a href=\"https://m3.material.io/get-started\">Material 3</a> design specification.<br /><br />Material 3 becomes the default Flutter interface in late 2023. To learn more about this transition, check out <a href=\"https://m3.material.io/develop/flutter\">Flutter support for Material 3</a>.",
+    "pagecontent": "To opt into Material 3, set the <a href=\"https://api.flutter.dev/flutter/material/ThemeData/useMaterial3.html\"><code>useMaterial3</code></a> flag to <code>true</code> in your theme.<br /><br />To catch these widgets in action, check out our live Material 3 <a href=\"https://flutter.github.io/samples/web/material_3_demo\" target=\"_blank\" rel=\"noopener noreferrer\">demo app</a>.<br /><br />You can still check out our legacy <a href=\"/development/ui/widgets/material2\">Material 2 widgets</a> over at their catalog page.",
+    "subcategories": [
+      {
+        "name": "Actions",
+        "color": "#D9E7CB"
+      },
+      {
+        "name": "Communication",
+        "color": "#F9DBDA"
+      },
+      {
+        "name": "Containment",
+        "color": "#F9DBDA"
+      },
+      {
+        "name": "Navigation",
+        "color": "#E5E4C2"
+
+      },
+      {
+        "name": "Selection",
+        "color": "#D9E7CB"
+
+      },
+      {
+        "name": "Text Inputs",
+        "color": "#E5E4C2"
+      }
+    ],
     "id": "material"
   },
   {

src/_data/catalog/widgets.json

@@ -0,0 +1,53 @@
+<!-- Material 3 Catalog Page -->
+{% for section in site.data.catalog.index %}
+    {% if section.name == include.category %}
+        {% assign category = section %}
+        {% break %}
+    {% endif %}
+{% endfor %}
+
+<div class="catalog">
+    <div class="category-description">
+        <p>{{category.description}}</p>
+    </div>
+    {% if category.subcategories and category.subcategories.size != 0 -%}
+    <ul>
+        {% for sub in category.subcategories -%}
+            <li><a href="#{{sub.name}}">{{sub.name}}</a></li>
+        {% endfor -%}
+    </ul>
+    {% endif -%}
+
+    <p> {{ category.pagecontent }}</p>
+
+    {% assign components = site.data.catalog.widgets | where_exp:"comp","comp.categories contains include.category" -%}
+
+    {% if category.subcategories and category.subcategories.size != 0 -%}
+    {% for sub in category.subcategories -%}
+        {% assign components = site.data.catalog.widgets | where_exp:"comp","comp.subcategories contains sub.name" -%}
+        {% if components.size != 0 -%}
+        <h2 id="{{sub.name}}">{{sub.name}}</h2>
+        <div class="card-deck card-deck--responsive">
+            {% for comp in components -%}
+                <div class="card">
+                    <a href="{{comp.link}}">
+                        <div class="card-image-holder-material-3" style="--bg-color: {{sub.color}}">
+                                {{ comp.image }}
+                            <div class="card-image-material-3-hover">
+                                {{ comp.hoverimage }}
+                            </div>
+                        </div>
+                    </a>
+                    <div class="card-body">
+                        <a href="{{comp.link}}"><header class="card-title card-title-material-3">{{comp.name}}</header></a>
+                        <p class="card-text">{{ comp.description | truncatewords: 25 }}</p>
+                    </div>
+                </div>
+            {% endfor -%}
+        </div>
+        {% endif -%}
+    {% endfor -%}
+    {% endif -%}
+
+    <p>Check out more widgets in the <a href="{{site.url}}/development/ui/widgets">widget catalog</a>.</p>
+</div>

src/_includes/docs/catalogpage-material.html

@@ -17,6 +17,8 @@
     </ul>
     {% endif -%}
 
+    <p>{{ category.pagecontent }}</p>
+
     <p>See more widgets in the <a href="{{site.url}}/development/ui/widgets">widget catalog</a>.</p>
 
     {% assign components = site.data.catalog.widgets | where_exp:"comp","comp.categories contains include.category" -%}

src/_includes/docs/catalogpage.html

@@ -63,6 +63,48 @@ dd {
   }
 }
 
+.card-title-material-3 {
+  color: $site-color-body;
+}
+
+.card-image-holder-material-3 {
+  align-items: center;
+  z-index: -1;
+  background-size: 0%;
+  opacity: .999;
+  background-color: var(--bg-color, white);
+
+  img {
+    max-width: 100%;
+    max-height: 100%;
+    width: auto;
+    z-index: 1;
+  }
+}
+
+.card-image-material-3-hover {
+  position: absolute;
+  top: 0;
+  bottom: 0;
+  left: 0;
+  right: 0;
+  opacity: 0;
+  transition: .25s ease;
+  z-index: -1;
+
+ img {
+    max-width: 100%;
+    max-height: 100%;
+    width: auto;
+   z-index: -1;
+
+   }
+}
+
+.card-image-holder-material-3:hover .card-image-material-3-hover {
+  opacity: 1;
+}
+
 .happy-paths-image-holder {
   align-items: left;
   display: flex;